1. Field of the Invention
The present invention relates to a water heater and, more particular, to a water heater for producing hot water with recovering heat from combustion gas generated by a burner.
2. Description of the Related Art
The present assignee proposes one example of a water heater in Japanese Patent Application Laid-open No. 2004-44912. This water heater is suitable for recovering sensible heat and latent heat from combustion gas generated by a burner by using a heat exchanger including water tubes. When latent heat is recovered from combustion gas, condensate (that is, condensation water in condensing vapor) is generated on a surface of the water tubes by condensing vapor in the combustion gas. This condensate causes a gas-flow blocking in the heat exchanger. The blocking of gas-flow decreases the amount of airflow to the burner and it is difficult to burn fuel appropriately at predetermined air-fuel ratio. Therefore, a controller associated with the present water heater decides whether the blocking of gas-flow occurred or not. This decision process checks and decides the blocking of gas-flow in the following cases; for example, when the amount of airflow decreases lower than a predetermined amount of airflow and this state continues more than predetermined period, or when the temperature of the heat exchanger decreases lower than a predetermined temperature and this state continues more than predetermined period.
In this water heater, the amount of airflow generated by a fan is controlled to increase when the blocking of gas-flow occurs. Therefore, the air-fuel ratio is preserved at predetermined value. The pressure of the airflow blows the condensate off from the surface of the water tubes, which solves the problem of the gas-flow blocking.
In the conventional art, however, there are the following problems to preserve the air-fuel ratio at the predetermined value.
That is, there is a process to accumulate condensate gradually on the surface of the tubes after starting to drive the burner. The gas-flow blocking is occurring in this step, but the gas-flow area in the heat exchanger is decreasing gradually.
In the conventional water heater, the amount of airflow generated by the fan does not increase when gas-flow blocking does not occur. Therefore, the fan does not supply a sufficient amount of airflow in the initial operating step of combustion and condensate gradually generates and accumulates. This shortage of airflow is relatively small.
To solve the above problem it is required to adjust the burner under the conditions as appropriate as possible.